1. Field of the Invention
The present invention relates to a control of a motor for driving a throttle valve in an intake system of an internal combustion engine.
2. Description of the Related Art
In examples of using a step-motor for driving a throttle valve, there have been disposed various techniques of accurately performing the opening/closing of a throttle valve. For example, in Laid-Open Japanese Patent Publication No. SHO 56-14834, to cope with a problem that a spring for biasing a throttle valve on the closing side is increased in its biasing force linearly with the opening of the throttle valve, the drive frequency of a step-motor is reduced according to the opening of a throttle valve to obtain a suitable drive torque and retention of the stability.
The prior art technique described above can meet a load proportional to the opening of a throttle valve; however, it does not take any measure for a change in load in a transient period.
In particular, when a throttle valve is started to be open from the full-close state, a change in intake-air amount is transiently increased, which acts as a dynamic friction to the throttle valve.
As a consequence, when a load is largely changed in a transient period, a torque for turning a throttle valve becomes insufficient, and the desirable valve-opening drive cannot be obtained, so that there is the fear of causing the out-of-phase in a step-motor or the like.
A method may be considered to usually drive a motor with a torque corresponding to the change in load in a transient period; however, it increases current consumption thereby deteriorating responsiveness.